With its shuttles about to retire, the agency has offered $270m (£166m) of funds to four firms to help them mature designs for new orbiting vehicles. Blue Origin, Boeing, Sierra Nevada Corp and SpaceX hope to sell astronaut "taxi" services to Nasa by mid-decade.

SpaceX, which has garnered much publicity recently, is perhaps the most advanced in its plans. It has already flown a rocket called Falcon 9 and a capsule called Dragon. It is being offered $75m over the next year if it meets certain milestones in advancing Dragon's crew-carrying capabilities.

The long-established Boeing company stands to win the largest award depending on developments. It has a capsule design called CTS-100 which could transport up to seven astronauts to the space station. The $92.3m Nasa support will help Boeing get the vehicle through to its preliminary design review.

* Sierra Nevada Corp. of Louisville, Colo., snagged $80 million to further develop its Dream Chaser vehicle, a seven-person spacecraft, to a preliminary design review stage. Dream Chaser is a winged spaceship designed to launch atop an expendable rocket.

* Blue Origin of Kent, Wash., will receive $22 million to further the development of its space vehicle design and pusher escape system. Blue Origin was established by Amazon.com founder Jeff Bezos and is developing the cone-shaped vertical launch vehicle New Shepard.

Nasa is keen that the next era of human spaceflight include a strong commercial element. It plans to substantially increase its seed funding in 2012.